nfsd: Ensure that nfsd_create_setattr commits files to stable storage

Since nfsd_create_setattr strips the mode from the struct iattr, it
is quite possible that it will optimise away the call to nfsd_setattr
altogether.
If this is the case, then we never call commit_metadata() on the
newly created file.

Also ensure that both nfsd_setattr() and nfsd_create_setattr() fail
when the call to commit_metadata fails.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>